I'm not sure if it was mentionned already, but space rangers 2 might be a good pick (can't say about the first as I never played). In the game, the 5 main races are at war against the dominators, a race of sentient robots. As the game progress, systems are invaded/liberated by the dominators. You play a ranger, a lonewolf pilot, and are free to do whatever you want: completing missions, trading, being a pirate, fight the dominators, etc. The war won't wait for you, so at high difficulty levels, if you don't help defending, you can end up with a galaxy controlled by robots.
The economy isn't bad at all either. Besides the typical (and boring
) point-to-point trading, you can invest in new military ships for a system, the construction of new starbases and that kind of thing. Also, the parts available to equip your ship will change as time progress in game; initially pretty much never find better weapon than missile launcher, but eventually you can turn a small spacecraft into a
magma cannon weapon of mass destruction.
There are other rangers out there that you can recruit or kill... or recruit and lead to their death. They all do their own things (same freedom as you), but a bunch act mostly as scavanger looting anything and everything left behind in invaded/liberated; which is IMHO the most annoying thing in the game.
Anyway, there would be more to say about this game, but this wasn't meant as a full review. Oh! However, I can't help adding that the galaxy/sectors are randomly generated which make every game different (replayability++).